OAKLAND — Delays plagued BART commuters for much of Friday after a device that monitors train speeds malfunctioned just outside the West Oakland station, but trains were mostly running on time by the heaviest portion of the evening commute.

According to BART spokeswoman Alicia Trost, the device, known as a muxbox, malfunctioned around 9 a.m., the height of the Friday morning commute. By noon, crews were out on the tracks to troubleshoot the system to get it up and running.

“The boxes do go down from time to time,” Trost said. “When it goes down during major commute hours, we’re going to feel it throughout the day.”

Crews worked along a three-mile length of track to repair the muxbox, which sends speed codes to trains traveling over tracks, Trost said. Trains began normal service around 4 p.m., though slight delays were expected for trains traveling back into San Francisco. Some trains were not stopping at the West Oakland station, Trost added, as crews finished up repairs and checks for the night.

“Riders need to be paying attention,” Trost said. “This is maybe not the time to listen to music on your phone.”

Trains are still traveling at reduced speeds through the Transbay Tube, but service is expected to run smoothly the rest of Friday.

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